"I did not sign anything”: can a tacit modification constitute a contract amendment under EU Procurement rules?

Public contracts and other types of commercial contracts share common grounds but also deviate from each other. In a private contract setting, the law protects the privity of contract and contractual freedom and does not allow third parties to challenge the contract's existence. In the public procurement context, private contract law rules are applied to the contracts but with limitations and modifications. Private national law rules apply particularly in the private-oriented hybrid model but only when they do not endanger the effectiveness and interests of the EU Public Procurement Law.
Since the Succhi di Frutta and Pressetext rulings of the CJEU, and especially since adopting the 2014 Public Procurement Directives, contract modifications have probably been one of the most discussed topics in the procurement research literature and practice. However, tacit modifications are one of the overlooked topics regarding contract modifications in the procurement context. Our research aims to answer the question of whether a tacit modification can constitute a contract amendment under the EU Public Procurement Law and what the impact of contract law rules is in that analysis.
The chapter concludes that the EU Public Procurement Law has precedence over private law. The 2014 Public Procurement Directive states that public contracts are to be drafted in writing but does not set any form requirement for modifications. Instead, in private law, as a starting point, contract modifications are required to respect the same formal requirements that the contract may have been subject to. However, this is not absolute, and other amendments can occur in private contract settings if parties accept them. Moreover, whether the written form of a public contract is an actual formal requirement is ambiguous. Recent EU case law suggests that characteristics of contract amendment include a will to renegotiate a contract, which can be demonstrated, among others, by written elements. Thus, there could be evidence of the existence of this will in other forms as well. In such a case, tacit modification would be possible – at least in theory. Another question is how the existence of mutual will can be demonstrated in such a case. Based on the wording by the CJEU, it appears that some demonstration of the will to renegotiate is required, and it cannot be assumed. But in the digital era, it is perhaps rare that there would be amendments that leave no written trace.