In relatively new loans into Finnish (for the last 1500 years, at least; AFAIK similarly in most other Uralic languages), *s+stop clusters are uniformly retained medially (piispa “bishop”, masto “mast”, viski “whisk(e)y”) and simplified to the stop initially (piikki “spike”, tyyli “style”, kuuro “shower”).
This has not always been the case. In sufficiently old Baltic and Germanic loans, a word-initial substitution *st → /s/ appears instead: e.g. Baltic *staibas → seiväs “pole” (interestingly, in the more southern Finnic languages the subtitution is already t-: Estonian teivas, Livonian tāibaz), pre-Germanic *starrā “rigid” → sara “sedge” (= “rigid grass”). This pattern is also found beyond Finnic: e.g. Samic *soampē “ski pole” derives from Proto-Indo-European *stombʰos “pole”. (Several IE branches could have been the actual donor, e.g. Balto-Slavic. The Samic word has also been further loaned to Finnish as sompa “stopper in a ski pole”.)
I’m not aware of particularly old estabilished cases of *sp- → *p-, *sk- → *k-. Or even many medial examples of any sort, beyond the PU-PIE comparison *mośkə- ~ *mozg- “to wash” (for which a loan etymology proposal would be complicated by the unmotivated palatalization on the U side). What if a substitution *sP → *s was originally employed even in more general? There is even a strange absense of *sp in the older stages of Uralic languages, despite ever-present Indo-European neighbors and the existence of *st and *sk in the inherited Uralic vocabulary.
At least one case well analyzable as *sk → *s can be found, I think. This is *sitta “shit”, with an immediate resemblance to its English translation. At the Proto-Germanic level, we find a long-vocalic *skīta-, with the desired *a-stem. The short vowel in the English noun is a late development (cf. German Scheiße(n-), Scandinavian skit(a-)), but for a sufficiently early loan, this is no problem: long vowels likely did not exist in Finnic at this stage, and in particular before open vowels they are considered a quite late introduction. Also the Finnic root-type *i-a is typical of loanwords anyway; no clear Uralic origin has been identified for it (though a few cognate sets do reach all the way to Ob-Ugric).
Aside from the unusual initial correspondence, the loan could be dated even as fairly late within the Proto-Finnic period. However there’s reason to consider it to be on the older side, which also grants some more space between it and the first clear cases of *sk → *k: the existence of a Permic cognate *sit (→ Komi /sit/, Udmurt /siť/). Direct Germanic loans in Permic are of course not an option, but precedents are known for Gmc → P transmission anyway, likely mediated by some early eastward offshoot of Finnic.
A slight problem may be the vowel correspondence F *i ~ P *i. Other known cases of loans from the extinct easternmost varieties Finnic into Permic have rather shown a correspondence *i-a ~ *ɨ (e.g. *liiva ~ *lɨa “sand”, originally from Baltic). Yet, the very oldest IE loans of the *i-a type do not show any such backing: consider the Iranian loans *wiša “green; poison” → Finnic *viha “hate”, *viherä “green” ~ Permic *vež (→ Komi /vež/, Udmurt /vož/) “green” , and *iša “skin” → Finnic *iho “skin” ~ Komi /ež/ “inner side of leather”. Not only do these retain the original front vowel, they appear to even be sufficiently old to have participated in the Permic lowering of *i in open syllables. (These also have reflexes beyond Permic. They’re not relevant here though.) If *sitta were to be of compareable age, *sit would indeed be the expected Permic reflex. Possibly the main problem in this scenario rather becomes if the Germanic word can be projected this far back?
I have another candidate for *sk being reflected as *s too. A root *sumə ~ *sumu “fog” is found in the westmost Uralic: Finnic *sumeda “blurry”, *sumu “fog”, Samic *somō “misty weather”, and Mordvinic *suv “fog”. Of note here is the irregular retention of *u as *u in Mordvinic, which could be explained by this being a loan acquired after the lowering of inherited *u. This same correspondence is found in some other words of limited distribution as well, particularly interestingly Erzya /ruŋgo/ ~ Mokša /roŋga/ “body”, which has been compared with Finnic *ruŋko “stem, body”; which in turn has been explained as a Germanic loan.
A reasonable loan original is here provided by Germanic *skum- ~ *skūm- “darkness” (cf. German Schummer, Scandinavian skum). A similar assumption of vowel shortening upon loaning as before may be necessary, depending on which Germanic variant is older. The final labial vowel (found in both Finnic and in Samic, and I wonder if even the irregular /v/ in Mordvinic could reflect the same) must be analyzed as a suffix added on the Uralic side.
I have not identified any cases of *sp → *s. This also seems somehow less likely: [p], as a labial consonant, is articulated more separately from [s] than [t] and [k] are. In fact I even have some proposals waiting for old IE loans with a different treatment of *sp. But once again, that shall be left to another post.