Did Paul abandon God's moral law when he became 'without law'?
No. Paul described becoming "without law" only in relation to those outside the law, yet he explicitly said he was not without law toward God but was under law to Christ. He never forsook God's moral law or treated grace as permission to do as he pleased morally.
Les explains that Paul's flexibility was not moral lawlessness. When Paul said he became as without law to those without law, he immediately qualified it: he was "not without law to God, but under the law to Christ." Paul never forsook the morality of the law of God, and he never claimed he was free from the law so he could do as he pleased in moral matters. Instead, he adapted himself to reach people, becoming all things to all men so that he might save some. Les illustrates this with missionaries who must live like the people among whom they work; they cannot simply impose Western culture, habits, and food, or they would never touch anybody.
To them that are without law, as without law, (being not without law to God, but under the law to Christ,) that I might gain them that are without law.