I am not an accountant, but let me explain what I believe the sentence means.
We are owed several sums of money, which we call the "amounts receivable"
However, we don't expect to be able to collect all of the money we are owed. The lesser total we actually expect to be able to collect is called the "net recognised receivable".
We calculate the difference between the two amounts, and then we apply it [probably as a percentage] as an "allowance against" any future amounts we are owed , so that we can anticipate how much we will actually collect.
In this context, "against" means "to be deducted from".
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