No, it is not a sin at all, and I would rather encourage you to be very careful of yourself and such requests.
Nobody can force you to have a child, let alone to give it away. That sounds preposterous.
After you decline, in the case your husband or anyone brings the topic up, or tries to guilt-trap you, inform them very sternly what your view on this is, and if it continues seek consultation.
Thank you for your question. The first step it to not be too harsh on yourself. Allah forgives all sins and most humans follow a path of mistakes before enlightenment. While you should avoid sin, even if you do sin don't be so hard on yourself that it effects your faith but instead seek a way back and try to reform. Seek forgiveness and make an intention to make meaningful changes and follow a good way. It is to then sincerely seek the truth and try and find it through knowledge and self purification until you manage to establish your feet on the straight path.
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