I mean, technically, you can write pretty much anything in your contracts. This is different because (in these cases) usual procedure would be to have loan fee and option to buy. But legally speaking penalty clauses in contracts are always there as a penalty if certain obligation that should be done in the future isn't fulfilled.
By some logic, in this case such penalty should be higher than let's say "common" loan fee for a high profile player (Kulusevski, Griezmann, Felix were all around €10m), but still slightly lower than what the obligation is. So, purely guessing it could be around 15 million pounds as a significant penalty for Chelsea not going through with the obligation and solid compensation for United which should still find ways to sell him to some other club after receiveing such amount. It is not ideal (if it even comes to this scenario) but calls for club being incompetent here are misplaced imo if Chelsea insisted on such clause and no other club was interested in buying or loaning Sancho considering his wages.
What is an example of terrible business while looking at loan fees in past few seasons was us sanctioning a €12m loan deal for Odion Ighalo. Or giving Sancho such wages in the first place.
Hopefully, Chelsea goes on with the obligation. It would say more than anything about Sancho that they are willing to pay a significant penalty not to have him as an asset at the age of 24. And they pay for feckin everyone.