Is one who said to his wife, regarding lengthening his trousers – provided they do not reach below the ankles – "I will see what I can do," in order to avoid them appearing too short according to custom, believing that the Prophet, peace and blessings be upon him, commanded shortening the garment to mid-calf, then sought Allah's forgiveness for his thoughts, sinful? And is it permissible to follow custom in this case as long as it does not contradict Islamic law?

The Prophet, peace and blessings be upon him, said that a believer's izar (lower garment) should reach the middle of his shins, then to his ankles, and whatever is below that is in the Hellfire. This applies to all clothing.

There is no sin upon you in what you mentioned, as you have not violated the Sharia. This issue is not about prioritizing custom over religion, because it is part of religion and was mentioned in the Hadith. What is forbidden is isbal (letting the garment hang) below the ankles.

It is permissible to follow custom if it does not contradict the Sharia. So, if the custom in your area is to wear clothing loosely above the ankles, there is no harm in it. However, if the custom encourages isbal below the ankles, it is not permissible to agree with it, as it contradicts the Sharia.