"Modern" safety regulations enforce that pressure gas cylinders which are only partially emptied cannot be returned to the supplier. This applies particularly for reactive gases.
In a pressure gas cylinder chlorin gas is found as a liquid. This means that even small pressure gas cylinders contain a lot of chlorin gas. This means also that a full gas cylinder is much heavier in weight than an empty cylinder. Since the weight of the empty cylinder is stamped on its top you may determine the available amount of gas by simply weighing the cylinder. A suitable balance is found at the "Materialverwaltung" (=equipment counter).
Escaping clorine gas is absorbed as described in "acidic reaction gas". There the chlorine gas will disproportionate to chloride and hypochlorite. The hypochlorite should be transformed to the chloride by adding a suitable reducing agent (for example disulfite). Check with iodide if this reaction has been completed. With larger quantities of chlorine gas it is best to add the reducing agent already to the absorption solution. Regarding to ecological concerns a very nice method is to reduce the chlorine with hydrogen peroxide because then the salt concentration is very low. According to V.Wiskamp (personal information) the hydrogen peroxide has to be added slowly to the hypochlorite solution until the formation of gas ceases. More acurate is to check if 0.1 % methylorange dissolved in 2 % sulfuric acid is not decolorised by excess sulfite.