When it comes to RPing, don't let the official rules stand in the way of logic and realism. For an assassination job, allow the PCs to be paid as soon as verification or proof of death is shown. In the BTU this could be as quickly as a few minutes after proof of death is shown and the employer is located on the same world or multiple months if the employer requires external (to the party) verification and exists on the other side of the sphere. Also, considering an employer for assassination is not going to want anything leading back to him, they may just take the mercenary's word for it, and if it turns out the merc was lying, well, that merc is likely to be the next target.