First I will add the same 'disclaimer' as PJRZ:
There are no official 5th edition rules about these kinds of specific situations. A DM is free to decide.
But fear not! The devil is in the details. My attention is at this part of the description:
[...] or dissolves into a pool of ichor.
You could rule that the ichor holds the devil's 'soul' until it leaves the warded area and returns back to Nine Hells. You can even make the ichor move towards an unwarded area until the planar travel is possible.
Something else to consider.
Under the Planar Travel (starting DMG p.44) there are only two ways for Planar Travel: spells and portals. Note both of these are magic. It's also fair to say when a spell or effect say Planar Travel is not possible they are talking about these 2 magical ways of travelling between planes. The description of devils however doesn't mention any spells or opening portals or anything about magic at all. Therefore you can also rule that this is not magical Planar Travel in the way the spells and portals work and let the poor thing go back to its home.