PureEngage On-Premises

When one or both are tracked for an exception and one is on time off.

It also depends on how you have the rules of the exceptions set up. You can set up exceptions to keep them with an agent, delete, keep with schedule or do not trade. If you decide to keep the exception with the agent and it falls outside of the new schedule, it won't allow the trade. It also won't allow them to trade without approval if they are trading a working day for a day off. 

Durga,

Trades will be blocked if the agents are not "compatible".
Compatibility is deemed by 2 things:  Activities and Contract Contraints.
The two Agents must be able to do the other's work on the day in question.  An agent who can speak only Spanish will not be able to trade schedules with an agent who can speak only English if those are skills that are relevant to their scheduled Activities.
The trade cannot violate either of the Agents' Contracts.  A Full-Time agent will not be able to trade with a Part-Time agent if the trade will violate their Min-Max hours for the week.

As Theresa stated, the presence of certain Exceptions in an Agent's schedule can prevent that day from being available for a trade.

Thank you Carla, Theresa and Thomas for your valuable answers, they were very helpful. Thanks again.

Can we consider this question solved?