A Devil’s guide to collecting souls
When a Devil is called to the material plane the Devil is accompanied by a Ink Devil so that there is "Accountable reference" and scribe for the deal (the deal is a limited wish spell based off the soul’s worth, so a lawful good paladin would be worth more than the chaotic good rouge.)
Once the Deal is established the Ink Devil has both the Devil and Summoner Sign the document (if the Devil has no way to sign the document another way may be used to seal on contract Examples: a kiss, firm handshake or a dance.) After which the Ink devil returns to its master and a copy is sent to Totivillus the Scribe of hell Where the document is stored.
The Devil is trapped in the circle until it is released or the circle is broken, if the circle is broken the devil is free to do what it wants but still cannot harm any humanoid it has made a deal with.
the Devil may remain on the mortal plane as long as it pleases (it does not require air, food or sleep.)
The Devil can make an unlimited amount of deals when summoned but will need to another Ink devil to formalize the contract
The Summoner must not be forced under duress when making a deal with the devil.
The magic circle will prevent the Devil from affecting the Summoners mind, hold the Devil in the circle, and give it disadvantage on attacks.
Note put in basic Contract form