can someone please do a grammar check or help me make changes to the letter?

Don't forget those capitals for the start of every sentence and for"I" referring to yourself.

'modifications' plural after 'all'

Omit 'due to the financial issue' You have already said that you don't have enough money.

'We are trying....'

"We agree to remove the gas pipe and heating pipe in the basement. we will also expose the plumbing, heating and the gas pipe for your inspection. However, I want to ask your permission to keep all the modifications that we already had been made to the unit.
In the meantime, we don't have sufficient money to make any changes to the unit. We will try our best to solve this problem with you, and I hope you can take my request into consideration.
Thank you."
I assume the person you are writing knows what you are talking about; it is not entirely clear to me.

We agree to remove the gas pipe and heating pipe in the basement and also expose the plumbing, heating and the gas pipe for your inspection. However, I want to ask your permission to keep all the modifications that we’ve conducted so far. In the meantime, we don't have sufficient money to make any changes to the unit due to the financial issue. We try our best to solve this problem with you and I hope you can take my request for consideration.

However I can make it better if you email me and tell me the main idea of this letter because it isn't clear.