Can a buyer negotiate to pay a working capital adjustment over time instead of all at once?

Yes, nothing requires a working capital true-up payment to be made as a single lump sum immediately once the final figure is determined. Like most other mechanics in a purchase agreement, the timing and manner of payment, whether a lump sum on a fixed date, instalments over a defined period, or netting the amount against another deferred obligation such as an existing holdback or vendor take-back payment, is something the buyer and seller can agree to and write into the agreement.

A seller may resist deferred payment on a working capital adjustment, since spreading it out adds the seller's own collection risk if the buyer's financial position changes before all instalments are paid, so this is a genuine point of negotiation rather than something automatically available once the calculation is final. If a buyer wants this flexibility, it is best raised and drafted into the working capital adjustment mechanics during the original negotiation, rather than proposed after the final figure is already determined and the seller is simply expecting to be paid in full.