Nay, we establish the law (αλλα νομον ιστανομεν). Present indicative active of late verb ιστανω from ιστημ. This Paul hinted at in verse Romans 3:21. How he will show in chapter 4 how Abraham himself is an example of faith and in his life illustrates the very point just made. Besides, apart from Christ and the help of the Holy Spirit no one can keep God's law. The Mosaic law is only workable by faith in Christ. 

Romans 4:1

What then shall we say? (τ ουν ερουμεν?). Paul is fond of this rhetorical question (Romans 4:1; Romans 6:1; Romans 7:7; Romans 8:31; Romans 9:14; Romans 9:30).

Forefather (προπατορα). Old word, only here in N.T. Accusative case in apposition with Αβρααμ (accusative of general reference with the infinitive).

Hath found (ευρηκενα). Westcott and Hort put ευρηκενα in the margin because B omits it, a needless precaution. It is the perfect active infinitive of ευρισκω in indirect discourse after ερουμεν. The MSS. differ in the position of κατα σαρκα.