Invoicing between partners is in general not allowed within Horizon Europe. According to the Annotated Model Grant Agreement of Horizon 2020, purchases between beneficiaries is not considered eligible as their is the risk of double funding or charging commercial profit margins. There is not explicit reference to invoicing between partners in Horizon Europe’ General Model Grant Agreement but we can confidently assume that this rule will not change due to the inherent risks it entails.
Other article of interest: Is subcontracting between partners allowed in Horizon Europe?
Specific cases (purchases):
Purchases between beneficiaries — Are in principle not accepted. If a beneficiary needs supplies from another beneficiary, it is the latter beneficiary that should charge them to the action. (Otherwise there is the risk that the grant is used to charge commercial profit margins.) Purchases between beneficiaries will only be accepted in exceptional and properly justified cases (e.g. beneficiary A is the usual supplier of beneficiary B for a generic consumable that beneficiary B needs for the action).