John Calvin Commentary Genesis 38:16

"And he turned unto her by the way, and said, Come, I pray thee, let me come in unto thee: for he knew not that she was his daughter-in-law. And she said, What wilt thou give me, that thou mayest come in unto me?" — Genesis 38:16 (ASV)

What will you give me, etc. Tamar did not wish to profit from prostituting herself, but to have a certain pledge, so that she might boast of the revenge for the injury she had received.

Indeed, there is no doubt that God blinded Judah, as he deserved. For how did it happen that he did not recognize his daughter-in-law's voice, with which he had long been familiar? Besides, if a pledge had to be given for the promised kid, what folly it was to hand over his ring to a prostitute? I pass over the absurdity of his giving a double pledge.

It appears, therefore, that he was then deprived of all judgment. And these things are written by Moses for no other reason than to teach us that his miserable mind was darkened by the just judgment of God, because, by heaping sin upon sin, he had quenched the light of the Spirit.