A couple of thoughts.
First, we are obliged to go to confession as soon as is reasonably possible. If work commitments prevent it, we can pray an act of contrition and resolve to confess our sins at the first opportunity, and we can know that we have God’s forgiveness. You could ask your priest for confession at some other opportunity, even if it’s in the sacristy 5 mins before Sunday mass. Once we have repented and resolved to confess our sins, we no longer have a mortal sin ‘on our conscience’, but don’t use that as an excuse not to go to confession.
I am told, though I don’t have the exact canon to quote (maybe someone can provide it) that it is even licit to receive Holy Communion if we have repented of a mortal sin but haven’t had the opportunity to confess it, as long as we then confess at the first opportunity, and not more than 28 days after.
There is no such teaching. You are probably thinking of Canon law 916 which says-
"Anyone who is conscious of grave sin may not celebrate Mass or receive the Body of the Lord without previously having been to sacramental confession, unless there is a grave reason and there is no opportunity to confess; in this case the person is to remember the obligation to make an act of perfect contrition, which includes the resolve to go to confession as soon as possible." [Canon Law # 916]
Grave reason is like if you are on the battlefield or in danger of death or traveling to country without a Catholic Church or you are the only priest who can celebrate the Mass…
Second, another option might be to phrase your request for time off in terms that would not strictly be a lie, but would help your (non-Catholic?) boss to understand the gravity of your need for confession. For example, ‘I have been having some health problems, and have an appointment to see a specialist’ or ‘I need to have some important repair work done, and this is the only time the man could come and see me’. Both are kind-of true metaphors of the work that is done in confession, and would constitute ‘mental reservation’ rather than an outright lie.
God bless you for your devotion to the sacrament of reconciliation.