Short answer
Mahr is a marital gift owed to the bride, while wedding expenses are separate costs associated with the wedding and celebrations.
Islamic Guidance
What Islam teaches
Mahr is established as a right of the wife. It should not be confused with dowry or general wedding expenditure.
Practical Guidance
What you can consider
Clearly record the agreed Mahr and separately discuss other voluntary expenses and wedding arrangements.
Sources
Qur'an 4:4; Qur'an 4:20.