A compromise solution would be to allow it but settle on a fair price formula and then apply a penalty to FFP with a multiplier. For example if the donor club bought the player for 40m, then take that value divide by the years to amortize it (say 4 years at 40m, 10m per year) and require the recipient club to incur an FFP penalty equal to that times a factor for the financial year of the loan. If the factor in the above example is x2 then it's minus 20 million towards FFP for the year (10m x2).
Hell, you can make the factor even scale based on the last rank of the club so the more successful a club becomes the more associated loans cost towards FFP.
RB keeps maiming club logos everywhere they go. What an eyesore.