Often when people subscribe for a membership, they want to have the opportunity to pay off the entire membership at once, or pay a bit in advance.
If you need to generate a membership-invoice in the future for any reason, you can do this is in the client's contract.
Go to the clients and staff app and click on "products" in the client's profile. Here you click on the membership which will bring you to the client's contract. At the top right of the screen under options, you can choose to "generate next invoice". If you've just attached the membership, you will also have the option to "pay complete membership". This will create one invoice for the remaining months on the contract.
After creating an invoice for a future date, you can modify, credit or pay it.