Some of the fees were received in 2017 which is the tax receipt they are entitled to for their tax return. Some of the fees (the ones you are having trouble receiving) are for 2018. If you issue one receipt for the combined amount, that means you will have to declare all of this money on your 2017 tax return. We have to pay taxes and issue receipts based on when the money was EARNED not when it was received. It makes absolutely no sense to include the 2018 fees with the 2017 receipt because then you must pay tax on them before they were due or received.
If you get audited this year, having 2018 fees with 2017 income is a sure fire way to get a detailed audit.
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