Depending on the quantity of water, it can harm your effluent by lowering your F:M ratio and MLSS.
A lot of SBR's are designed for peak wet weather flows considerably greater than average however, and it might not be an issue. You need to look at the design conditions of your facility and your current average flow conditions.
If you are not near capacity at your facility, you "might" accept the water under conditions (ie limit flow, limit timing of release, etc.). You don't want the excess water during storm events when you already get peak flows.
I would be very concerned about what they are going to give you. Make sure you have testing performed routinely on the water.
Why is this water bad that it can't be discharge directly. Are their VOC's in the water? Are they filtering the water. Can they filter the water themselves then discharge it directly?
I'd have a lot of questions before accepting what I can't give back. If you discharge something bad, it is your butt in the sling. You accepted it and then you discharged it.
Be careful.