I bought a house with 2 units in Switzerland recently for $750k mainly for rental income and long term security.
The house returns $29,000/annum brutto rental
In the very long term, house prices appreciate by inflation (2% per year). However, that's for a portfolio of houses part of which are demolished and part of which are newly built. So, a year from now, the average age of houses has stayed the same because some old houses were demolished and new houses were built. The age of your house is 1 year more after 1 year from now.
If you want your house price to appreciate, you need more than minimal renovation costs. In fact, you should spend about 2% of the construction cost of a new house (not including cost of land) for renovation. I don't know how much of the $750k is land and how much is the building on the land, but let's be generous and assume 50%/50%. So, you should spend $7500/annum for renovation based on that.
The real yield is $29 000 / annum rental minus $7500 / annum renovation, giving $21 500 / annum.
You have to subtract heating costs, management costs and taxes too. I don't know what those are for you so let's be generous and assume all are $0 (unrealistic assumption).
Even given this unrealistic assumption, the real yield is 2.866%.
With a more realistic assumption, I suspect the real yield would be below 2%.
For a well-diversified stock portfolio, you get about 4% dividend yield plus about 2% economic growth in the long term giving 6% real yield.
In contrast to a well-diversified stock portfolio that is almost guaranteed not to fail, an individual property is subject to many risks. For example, what are you going to do if a water damage and mold problem destroys the value of your investment? An investment to an individual asset always has risks. With houses, you cannot diversify with minimal money; with stocks, you can.
Also, house prices can and will crash.
The largest decline in real prices was from 317.9 (in 1778/9) to 68.1 (in 1814/5), meaning the real price went 79% or approximately 80% down. Note that's for a well-diversified house portfolio. An individual house can decline 100% in value quite easily.
With stocks, the largest decline (during Great Depression) was similar, 80% down.
So, having established that the risk in a well-diversified house portfolio is the same as in a well-diversified stock portfolio, and having established you obtain 2-3% real yield from your house as opposed to 6% real yield from stocks, I'd say the answer is pretty clear.
Your property investment does not make sense even given the $750k price
You can more than double your real return AND reduce your total risk by obtaining a well-diversified stock portfolio as opposed to house investment in this very overheated housing market you are currently investing in.
I would sell the investment immediately, no questions asked, even if someone offered me only $500k.
You got a very good offer. Now is the time to exit.