Hmm this a tough situation due to the fact that you are sealed up inside with the RH outside being so high...
50%RH is high for flower cycle once buds develop. 50% is okay for the first week or two. Week 1-2 50%, week 3-4 40%, week 5-6 30%, and try to drop the RH a low as possible for the final week or two as the buds will try to protect themselves from the low RH by forming MORE RESIN glands AKA more Trichomes. Most growers bottom out around 25% or 20% RH.
How will you accomplish this? HMMMM If you want to save your cannabis crop perhaps you will need to sacrifice the tomatoes. I say that because you want to remove as much wet media (aka soil in pots) as possible to reduce the overall amount of moisture inside.
Try to only water when the sun/lights first come on. This way the plants have all day to drink the water and the pots should be dry before the lights go out or before the sun goes down. Not too dry but dry enough so that your plants will be ready to water first thing when the lights come back on. Remember they won't be drinking with the lights off specially if the RH is 90% they will not drink at all like that. So make sure those pots are dry just before lights out. This way you do not have wet media in your green house in the dark when the RH outside is already so high.
As the previous poster said, artificial heat can dry out your environment. An electric space heater that uses a hot coil with a fan blowing over it could work to drop the RH a few points depending on how powerful the space heater is. That said, you will want to monitor the temperature because you would never want to overheat your grow space for the sake of dropping the humidity. However, plants can generally tolerate heat well when everything else is on point - so you could get away with temps as high as 85f but at the same time you want your daytime temps to be slightly higher than your night time temps.
No real easy way out if you do not want to spend money on more dehumidifying power. Some pictures could help us advice you on your HVAC situation.
Generally when the RH is too high inside the grow space you would exhaust all that air out but ONLY IF the incoming air itself is not too humid. Is there any way you can intake air from a dry source? If your grow space is close to a house or a garage where the RH is lower, that could be a source of dry air. Something to think about..