Let's say I have a company and I own 100 % of it.
An investor wants to invest $1000 and gets 10 % of the company, I now own 90 % of the comopany.
To who pertains the $1000 - to me or to the company?
If the investor bought newly-issued shares or treasury shares, the company gets the money.
If the investor bought shares already held by the owner, the owner gets the money.
A 100% owner can decide how to structure the sale. Yet, the investor may only be willing to buy shares if the funds increase the company's working capital.
There is a legal document called a "Stock Purchase Agreement" and it depends on who is the other party to the buyer in the Agreement. In almost all startups the sale goes through the company, so the company keeps the money. In your example, the company would be worth $10,000 "Post-Money" because the $1k got 10% of the Company.
In my mind you would get all the money.
You owned 100% when that transaction occurred. S/He gets 10% then on everything after.
I usually go to an extreme case to figure out the answer. So...
If S/He bought 100% of the company it wouldn't go to the company it would go to you.
I would be open to criticism on this answer I am answering from common sense not because I really know the answer.