أهلية الشخص البالغ من العمر 19 عامًا للتوقيع على مذكرة تفاهم

According to UAE law, a person should be 21 years old to sign a contract. 18 years old person can sign with the consent of a legal guardian.

Article 160 of the civil code:

(1) A guardian may permit a minor who has reached the age of 18 Hijra years to receive all or part of his property to administer it.

(2) It shall be permissible for the court upon hearing the evidence of the guardian to permit a minor who has attained the age of 18 Hijra years to receive all or some of his property to administer it.

As you mentioned that you have signed an MOU with the purchaser, MOU is not a legally binding document, and you legally can't sign a contract which means that any paper was signed with the buyer is void. So do not worry, you don't need to pay anything to the buyer.

For further assistance, you may contact me directly via phone or email.