fix (IRC § 22(k))

The agreement must expressly specify or 'fix' a sum certain or percentage of the payment for child support before any of the payment is excluded from the wife's income. The statutory requirement is strict and carefully worded. It does not say that 'a sufficiently clear purpose' on the part of the parties is sufficient to shift the tax. It says that the 'written instrument' must 'fix' that 'portion of the payment' which is to go to the support of the children. Otherwise, the wife must pay the tax on the whole payment.