But what if the contract is "I will give you $X in three months time in exchange for Y% equity in this entity"? It's not unusual to enter a contract that oblige

But what if the contract is "I will give you $X in three months time in exchange for Y% equity in this entity"? It's not unusual to enter a contract that obliges you to pay money you do not yet have at some future date when you do expect to have it. If your contract system doesn't allow that then it isn't a very useful contract system.