You should seek guidance from a tax professional.
It is unlikely that can deduct the entire differential in the same tax year as an expense.
Are you in business or is this personal?
If you are in business, there are two possible ways I can think of to report this. Remember that business assets must be placed in service and depreciated over their useful lifespan. So one method would be to report the grant as gross income, and place the greenhouse in service as an asset with a cost basis of $10,200. A second method would be to not report the grant as income, and to place the greenhouse in service as an asset with a cost basis of $2731.
There is a separate question of whether you could expense the $2731 instead of depreciating it. The cost exceeds the $2500 safe harbor for expensing assets, but it might be allowable under a different safe harbor where called the safe harbor for small taxpayers, where the cap is 2% of the property's cost basis or $10,000, whichever is less, taking into account the total of all your expenses for maintenance, repairs and improvements. (In this case, property means real property, which is land and anything permanently attached. If you have a nursery business, for example, and add a greenhouse to your land, the greenhouse becomes part of your real property and must be depreciated. Whether or not you can use the safe harbor for small taxpayers depends on the cost basis of the rest of your land and other improvements, and the amount you spend on maintenance and repairs for your land and other improvements.)
So it depends on the type and size of your business and your other expenses. I agree about getting professional advice if you want to expense the difference.