I have made an oath to refrain from repeating a sin where I swore that everytime I repeat I must pay $Y to charity.
Although after a while this became a high load which will put hardship on me to pay and I won't be capable to fullfil it due to needing the money.

1. Is the oath still valid?

2. If the oath is not valid due to these circumstances and at a later date I become capable must I fulfill the oath?

1- If you are unable to pay the amount, then it’s ok you won’t be sinning. However, whenever you get the financial ability you must pay it.

2- Yes you must fulfill it whenever you have the financial ability