Units that reduce their emissions below the number of allowances they hold may trade allowances with other units in the system, sell them to other sources or save them for future use.
Units may not sell emissions to other sources. 
mnli hypothesis: Units may not sell emissions to other sources.  premise: Units that reduce their emissions below the number of allowances they hold may trade allowances with other units in the system, sell them to other sources or save them for future use.