đĄ You can't top up a grant by donating to the organization. If you need a little more, ask the program to add funds to your grant, or buy the item yourself and request a reimbursement.
Why you can't top up a grant yourself
A grant card isn't a personal bank account. Donating to the organization won't add to your grant, so any extra money has to come from the program that issued it.
Two ways to cover a little more
Ask the program to add funds. The program that issued your grant can increase your grant amount from their own program.
Pay the difference yourself, then get reimbursed. Buy the item with your own money and request a reimbursement from the program. You'll be paid back up to the grant's amount, and anything beyond that is on you.
For how reimbursements work, see how do I get reimbursed for a grant purchase.