Put option - exercise when it expires but don't have enough money to buy stocks to sell

Most brokers will simply give you the cash difference ("cash settlement") when the options expire in the money.

Also in most cases, you don't need to exercise the options to cash in. You can just sell them at any time. This is true whether the options are in the money or not. They'll have some market value, that may be higher or lower than the premium you originally paid for them. Basically, you can trade options just like stock.

US centric answer. Find out if any of this applies to you in Germany:

Long options can be sold to close on the option exchange. In most cases, that makes more sense because you don't throw away time premium (if any remains) and you don't need a bucket of money to exercise and then close the underlying (for cash or marginable securities).

Early assignment can create account violations if you don't have the available margin. Brokers will not just credit you the difference.

The broker Robinhood is a different beast. If you don’t have enough buying power to exercise your in-the-money option, they’ll sell it about 1 hour before it expires. Since ITM options tend to have wide bid/ask spreads, that never works in your favor since they trade at the market. It's a rip.

As others explained, you should sell the options on the last day (or earlier) and cash out. You would get more than from an exercise, and take away all risk.

The consequences of you lacking margin / cash to exercise the options can have different effects, and they depend on your brokerage:

• they could exercise them anyway, leaving you with a lot of shares short, and a matching huge cash balance in our account. Then they will contact you and urge you to either provide more margin, or close your shorted shares, as soon as possible (maybe within a day). Happened to me before, pretty nerve-wracking the first time, but worked out ok.
• they could force-sell your options before expiry - no big difference to you selling them, expect you probably get a worse price, blowing several percent of your money away.
• they could simply not exercise them, and they go in the trash. You obviously don't want to end up that way!

Again, selling them before expiry is clearly the best for you, but if you insist on waiting out the deadline (because you like losing money?), make very sure you don't end up with the last option - talk to them early enough to make sure you like the result

Edit: maybe that is not clear to you, but an in-the-money option will aways trade for more than the amount it is in-the-money - so a put for 900 strike will trade for more than 500 if the shares are at 400. Selling the in-the-money option always gives you more that exercising it and closing the share position right away.