Seve, No, it's not. The relative pronoun 'which' is correct in your sentence. I have received an invoice of $100 on WHICH I had to pay $20 of tax. 'Which' refers to the "invoice of $100". "What" can be used as a relative pronoun but it can never refer to a noun or pronoun that comes before it. $20 is what I paid on the invoice of $100.......................correct $20 is the amount what I paid on the invoce of $100....incorrect $20 is the amount that I paid on the invoice of $100.....correct I already spent what I earned last month......correct Nothing can change the destiny what is mine.....incorrect Nothing can change the destiny (which or that) is mine....correct Love is what can change my destiny..........correct