Workplace type: Remote-first (with on-site team meetings twice a month in Warsaw)Contract type: B2B / Permanent (UoP)Salary (B2B): 100 – 110 PLN + VAT / hourAvailability: ASAP/ up to 1 monthAbout the roleWe are looking for a Regular .NET Developer to join a team responsible for developing and maintaining core card payment systems - from transaction authorization and settlement to integrations with external payment services.The systems process hundreds of thousands of transactions daily, so performance, availability and security are key priorities.Work model: Remote-first with scheduled on-site team meetings (2 days per month in Warsaw).Responsibilities Develop and maintain .NET applications in card payment domain Design and implement new functionalities according to business and regulatory requirements Integrate systems with external services (REST, SOAP, messaging queues) Optimize performance, availability and security of applications Cooperate with Analysts, Architects and QA in Agile/Scrum teams Participate in SIT/UAT testing, code reviews and preparation of technical documentation Required skills Commercial experience with C# and .NET (Framework / Core) Experience with ASP.NET Core, Angular and WebAPI Knowledge of Entity Framework, LINQ, Dapper Experience with SQL Server (T-SQL) and query optimization Knowledge of SOLID principles and design patterns Experience writing unit tests (xUnit / NUnit) Experience with system integrations (REST, SOAP, JSON, XML) Experience working in Agile environment Knowledge of Git, CI/CD and TeamCity Language requirements Polish: native or C2 level (required) English: professional level enabling work with technical documentation Nice to have Experience in card payments / payment processing domain Knowledge of Docker / Kubernetes Experience working on high-availability or complex distributed systems Margo Offers: Long-term cooperation on stable banking project Remote-first work model with regular on-site collaboration days Employment or B2B contract (based on your preference) Benefits such as medical care and sports card Opportunity to grow in enterprise-scale systems Collaboration with experienced developers and architects Friendly and supportive MARGO team