It'd have to be negotiated if it was to be paid in installments, vs being triggered directly with the lump sum. Otherwise you could theoretically have a team bid a quid up front, with the rest of the fee spread in installments and the selling club would be forced to sell if the total fee hit the clause.
With that said, there'd definitely be negotiations around a release clause if the player wanted the move, or if the terms of the clause have set financials within it re: payment structure.
That's my understanding anyway - take with a pinch of salt!