Is it permissible to give half of the promised amount in cash and the other half in food supplies to the poor, after having intended to donate 500 riyals upon your wife's pregnancy, but you do not recall the specific method of giving the charity?

A vow is discouraged, but if one vows a vow that is considered an act of obedience to Allah, then he must fulfill it. Since you have vowed to donate a certain amount of money, you must fulfill it by distributing it to the poor, and not by buying a sacrifice or anything else with it.