(a) 100 percent tax on prohibited transactions (1) Tax imposed There is hereby imposed for each taxable year of a REMIC a tax equal to 100 percent of the net income derived from prohibited transactions. (2) Prohibited transaction For purposes of this part, the term "prohibited transaction" means- (A) Disposition of qualified mortgage The disposition of any qualified mortgage transferred to the REMIC other than a disposition pursuant to- (i) the substitution of a qualified replacement mortgage for