Word for calculating/adjusting a payment when I previously paid too much?

As part of a percentage rent, I need to pay my landlord 10% of my monthly revenue, but no lower than $10,000.

Therefore, I automatically pay $10,000 at the beginning of each month. So let's say we're dealing with January. I pay $10,000, and shortly into February, I have determined my monthly revenue, and 10% of it is $15,000. Now I need to pay an additional $5,000 for January, instead of all 10% of my revenue (because I already put down $10,000).

Now, what is the word for what we did to the 10% of revenue to use only the portion needed to leave the landlord with only $15,000?

Example sentence:

Because I have already paid $10,000, we must _________ the $15,000.

The word offset has been suggested, but I do not think it is an accurate choice.

Also - please make suggestions that fit in the example sentence above, the structure is quite important to my question.

You deduct the advance payment ($10,000 in this case) from the final/total rent payable.

Because I have already paid $10,000, we must deduct it from the total ($15,000).

ODO:

deduct VERB

[WITH OBJECT] Subtract or take away (an amount or part) from a total:
‘tax has been deducted from the payments’

‘Any severance already paid to the workers will be deducted from that amount, the judge ruled.’

TFD(idioms):

deduct (something) from (something else)
to subtract an amount from another amount.
Mr. Wilson deducted the discount from the bill.

McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.

Wiktionary:

Verb deduct (third-person singular simple present deducts, present participle deducting, simple past and past participle deducted)
To take one thing from another; remove from; make smaller by some amount.
I will deduct the cost of the can of peas from the money I owe you.

... we must pay the balance.

See also this entry.

Offset is fine, but you don't offset the $15000. You offset the $10k against that sum.

Because I have already paid $10,000, we must offset it against the $15,000.

Oxford's definition isn't particularly helpful, but there is an example which matches, noting how tax liability can be reduced by charitable donations already made:

offset verb
[with object] Counteract (something) by having an equal and opposite force or effect:

‘donations to charities can be offset against tax’

Other examples, showing a slightly different usage:

• The cost would be roughly $1.5 billion, which he says would be offset with spending cuts. [online.wsj.com]
• The cost of the program was partly offset with a $1 million grant from Mercury Insurance. [dailynews.com]

In both usages, offset against and offset with, the larger sum is as stated and the smaller sum is offset.

The overage is exactly the term for the additional increment of rent that you must pay:

overage, 1: Rental increment tied to sales or profit that is in addition to the flat rental fee. Assume a retail store in a mall is charged a base rental of $5,000 per month plus 2% of sales. If sales are $100,000, the overage would be $2,000 and total rental would be $7,000.

So, for your hypothetical month of February, your overage is $5,000, your total rental is $15,000, from which you debit the $10,000 base rental already paid.

To fit your sentence:

Because I have already paid $10,000, we must debit it from the $15,000.

To fit more common usage, the sentence should use the form be debited somehow:

Because I already paid my base rent, $10,000 should be debited from this month's rent of $15,000.