Empty field to fill in for the price

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HI,
is it possible to have an empty field to fill in for the price of a product?
I manage a sports association where students can pay for the annual course or pay for the course in 3 tranches. I could then create 4 products.
Annual course 300 euros
First tranche 100 euros
Second tranche 100 euros
Third tranche 100 euros
The problem arises when a person arrives at courses that have already started. Let's simulate the example that you arrive exactly halfway through the course.
If he decides to pay everything in one lump sum, he should pay 150 euros (50% of the annual rate of 300 euros)
If he decides to pay in tranches, he should be able to purchase the first tranche for 100 euros and 50% of the second tranche.
The only solution I've thought of is to provide a personalized coupon.
However, I ask you if there is the possibility of creating a product with a free price field that can be filled in by the student. In this case I could provide the student with the cost to enter, based on the period he still has to use.
Or do you come up with other solutions?
Thank you
Francesco

Thank you, best regards
Francesco