@Sofia Duque There is some critical information left out needed to make this decision.
Is $900/month rent upcharge reasonable or at market rate? Have you thoroughly vetted the new tenant?
If the answer is 'no' to these questions I would be cautious. When I get people that say something similar to the following I find it a major red flag and always proceed with extra caution.
"I'll pay $XXX over what your are currently asking"
"I have cash in hand"
"I'll pay 3 months, 6 months, full year rent in advance"
These are often tactics that historically bad tenants use to get your guard down so you don't do your due diligence.
My philosophy is to always try to retain tenants who have proven to be a good tenant. You should let your current tenant know that they are well below market rent and that you need to raise the rent. If what you are asking is reasonable they will more than likely stay.
At the end of the day you get to choose how you want to do business. If you are focused on making maximum profit then that's what you do. I would caution that numbers on a spreadsheet aren't always how you determine maximum profitability. As a landlord you are in the people business. If you treat people like dirt, you often get mud on your face.