You can politely decline by thanking the person warmly and giving a simple reason without over-explaining.
Understanding hospitality
In Pakistan, hospitality is valued, and invitations are often sincere. A respectful response such as expressing appreciation and mentioning prior plans is generally well received.
Keeping it simple
There is no need to provide detailed explanations. A calm tone and friendly manner help maintain mutual respect.
Social expectations
Declining politely does not usually cause offense, especially when gratitude is clearly expressed.