Can you pay with houses or hotels or do they remain with you forever?
From the Monopoly Deal FAQ (scroll down to near the end):
Q - WHICH CARDS CAN I USE TO PAY OTHER PLAYERS? (E.G. FOR RENT, BIRTHDAYS ETC.)
A - You can choose to pay:
- from your Bank (with money or Action cards that you've Banked as money),
- with Property cards, or
- with a combination of both.
Remember, you can only pay with cards that are on the table in front of you, never with cards from your hand.
You cannot pay with Houses or Hotels you have on properties because they are not in your bank nor are they Property cards themselves. If you had Banked them, you could use them to pay for things.
However, if a player charges you money and you decide to pay with some of the property from your completed set that includes a House and/or Hotel, the House or Hotel must be placed on the table next to your property section until you complete another set and it can be placed on top. You can also pay with your house or hotel card if you choose.
The rules (which you can check on this Monopoly wiki page) say that you absolutely cannot play cards from your hand. You can put hotels and houses into your bank though.
I’m pretty sure you can pay with hotels/houses since they have an amount on them in the corner! We played a tutorial game here: https://www.100kelvins.com/blog/how-to-play-monopoly-deal-card-game/