Common terms of surrender (assuming the city surrendered early enough) would have looked something like this:

• The garrison will be allowed to live and march out of the city with their weapons, colours, families, possessions, etc. And go to the nearest friendly city

• The town and its citizens would agree to pay a monthly instalment to the occupying army, for its upkeep and to fund the war effort. In exchange for this instalment, the commander of the occupying army would issue a protection warrant for the town. This guaranteed the town protection from further harm and guaranteed the soldiers would be on their best behaviour.

• Also, the soldiers would be expected to buy stuff for themselves at fair prices, in addition to the rations they already possessed.

Now other things could happen that could violate this agreement or in some cases the people (both the occupiers and the townfolk) just didn't abide by it.

An example would be from a small town called Stargard in Northern Germany. A local soldier was killed by the townspeople and the commander of the occupying army tried to levy extra taxes on the town as punishment.

Also sometimes the leaders felt they needed more money and thus would try and extort more money from the towns. For example in 1642 the Swedish army kidnapped 42 hostages from Munich in order to try and extort more money from the city. In Mainz the Swedish forces threatened to burn a Jewish synagogue unless the local Jewish community could pay a huge sum of money.

This was how the various factions funded their armies. It was especially crucial because the various participants were not used to such prolonged fighting with such big armies. The Imperial Army (Holy Roman Empire) reached around 100,000 at its peak which was unprecedented for the time and thus the regular system of taxation couldn't keep up with these huge armies. The billeting system described above was actually the result of a mercenary commander named Albrecht von Wallenstein, who saw it as a way to keep large armies in the field without going into tremendous debt.

how confident could the people surrendering be about that the terms would be kept, instead of the besieging army now just slaughtering everyone?

Fairly confident, obviously there were examples of these truces not being kept and armies trying to extort more money. But if an army wanted to break a treaty, it was far more profitable to kidnap citizens for ransom and to extort money from them; rather than just sacking the town completely.