Credited to him as righteousness

"and being fully convinced that what He had promised He was also able to perform. And therefore, 'it was accounted to him for righteousness." - Rom. 4:21-22
My son in grade 3 has a system in class where they have certain duties to perform, when they perform those duties they get credits. However, if they are misbehaving or not accomplishing their duties, they get debits. So, there are debits which are taken off, and credits, which are added on.
At the end of a certain time period, they add the credits and subtract the debits and then the teacher writes a pay cheque which then they go to the bank (in the classroom) and cash it in.
Romans up to this point is talking about how the law points out our transgressions and that all have sinned and fall short of the glory of God. Abraham, is said to be a righteous man, however, it is not because he was a Jew or that he was circumcised, but it was because he believed that what God said was true and that He would carry out the things that he said. His faith is shown in the sense that he didn't even consider what doesn't make sense to us humans, but rather, had faith in God who made the promise.

Paul then goes on to point out that this is not only for Abraham but for us who believe in Jesus.