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).ResponsibilitiesDevelop and maintain .NET applications in card payment domainDesign and implement new functionalities according to business and regulatory requirementsIntegrate systems with external services (REST, SOAP, messaging queues)Optimize performance, availability and security of applicationsCooperate with Analysts, Architects and QA in Agile/Scrum teamsParticipate in SIT/UAT testing, code reviews and preparation of technical documentationRequired skillsCommercial experience with C# and .NET (Framework / Core)Experience with ASP.NET Core, Angular and WebAPIKnowledge of Entity Framework, LINQ, DapperExperience with SQL Server (T-SQL) and query optimizationKnowledge of SOLID principles and design patternsExperience writing unit tests (xUnit / NUnit)Experience with system integrations (REST, SOAP, JSON, XML)Experience working in Agile environmentKnowledge of Git, CI/CD and TeamCityLanguage requirementsPolish: native or C2 level (required)English: professional level enabling work with technical documentationNice to haveExperience in card payments / payment processing domainKnowledge of Docker / KubernetesExperience working on high-availability or complex distributed systemsMargo Offers:Long-term cooperation on stable banking projectRemote-first work model with regular on-site collaboration daysEmployment or B2B contract (based on your preference)Benefits such as medical care and sports cardOpportunity to grow in enterprise-scale systemsCollaboration with experienced developers and architectsFriendly and supportive MARGO team