In an era in which many armies were raised for a campaign, rather than being a standing army, the enemy's men might be willing to take your money to leave you alone & go back home, but they wouldn't necessarily want to join you & fight for your nation, especially against their own. A general who accepted such a bribe probably wouldn't want to show his face at home again, though.

Historically bribes were paid for relief to attacking armies. If you wanted extra troops, you had to hire your own mercenaries.