I think
@jojojo highlighted some of the challenges. As I understand it, the Spanish rule on buy-outs is ambiguous as to procedure. Someone has to buy-out the contract. Presumably if United transfer the funds to the LFA then Herrera can't be on the hook for taxes associated with the buy-out amount.
If, however, United proves Herrara with the funds-well the amount provided by United to Herrara could be considered taxable income-and Herrara would have to pay taxes on that amount.
Given tax rates-the tax liability on £36M is considerable. Not sure this is the issue-but could be a potential sticking point.